1. Field of the Invention
The present invention relates to a path switch ring (PSR) method in an SDH (synchronous digital hierarchy) network, and more particularly it relates to a path switch ring control apparatus which is capable of repairing a fault that occurs in a PDH subnetwork in the case in which a PDH (presiochronous digital hierarchy) subnetwork of a PDH network PDH is connected to the above-noted SDH transmission network.
2. Description of Related Art
In an SDH transmission network, for the purpose of repairing, for example, a circuit fault, an MSP (multiplex section protection) method which features redundant signal transmission lines is used, or the transmission path is configured as a ring, and a path switch ring (PSR) which has redundancy by the use of a VC (virtual container) is used. In both systems service is continued when a fault occurs in the transmission path, the MSP system being used in linear optical trunk networks between large cities, and the PSR system being used in networks performing complex transmission within a city.
In a path switch ring network, in the case, for example, in which service is being provided by two network elements (NEs) a and b via the respective SDH interface points on the ring transmission path, the transmitting-side NEa transmits the same signal to the receiving-side NEb both clockwise and counterclockwise on both VCs, and at the receiving-side NEb a path switching function is used to select the one of the given signals that has the better quality by comparing the circuit quality of each. In this case, even if a circuit fault occurs on one of the directions (clockwise or counterclockwise), service is continued because it is possible to select the other, normal, transmission path.
The SDH frame is the basic unit for processing of multiplexed, cross-networked digital signals, and is formed by a section overhead (SOH) of 9 octets and a 261-octet virtual container (VC). In a path switch ring (PSR) in the past, by monitoring a circuit quality monitoring byte (B3), a path trace byte (J1) and a signal label byte (C2) which are included in the path overhead (POH) of, for example, the virtual contain 4 (VC-4), the path having the better circuit quality was selected.
However, in the above-noted ring network, because a terminal on the VC path which uses a path switching function other than at the service access point was not allowed, a first problem arises in that, in the case in which an existing PDH network is used as part of a redundant route (clockwise or counterclockwise) of the SDH ring network, or if a redundant route is connected to a PDH network of a different network supplier, it was not possible to detect a fault which occurs within that PDH network at the SDH network side. In this case, it is not possible to select a proper path at the SDH network side, and an interruption of service occurred.
A second problem that arose was that, in the case in which an additional cross-connect apparatus (PDH DXC: PDH digital exchange) exists in the above-noted PDH subnetwork, if a path setting error occurs at that cross-connect apparatus, it causes an erroneous signal to be provided to the service access point on the SDH network side, resulting in even more difficulty in detecting the above-noted fault.
Yet a third problem that arose was that in the case in which an additional SDH ring subnetwork was encompassed within and connected to the above-noted PDH subnetwork, if a fault occurred in the PDH subnetwork, because the above-noted fault signal is sent in both the clockwise and counterclockwise directions on the SDH ring subnetwork, at the receiving-side network element (NE), the delay in the signals from each direction causes unnecessary path switching (PSW) or unnecessary notification of this event to a host apparatus.
Yet a fourth problem that arose was that, because of the connection between the SDH network and the PDH subnetwork, PDH/SDH multiplexing and demultiplexing on a path which passes therethrough exhibits mapping/demapping jitter, this jitter accumulating to cause degradation of the circuit quality.